Gas turbine engine turbines conventionally comprise axially alternate annular arrays of stator vanes and rotor blades. The rotor blades are usually mounted on the periphery of a rotatable disc adjacent the stator vanes. In order to ensure that rotatable and static components do not contact each other under normal operating conditions, an annular gap has to be provided between the stator vanes and the bladed rotor. This presents a problem, however, in that precautions have to be taken to ensure that the hot gases which normally pass through the turbine do not leak through the annular gap. Such leakage would be highly undesirable in view of the loss in turbine efficiency which would occur and overheating of the rotor disc and static structure adjacent to it.
The conventional method of addressing this problem of hot gas leakage is to supply high pressure cooling air to the gap between the stator vanes and the bladed rotor. Thus cooling air is directed radially outwardly over the surface of the rotatable disc and adjacent static structure to exhaust through the gap into the hot gas stream. Obviously for the cooling air to be exhausted through the gap its pressure must be greater than that of the hot gas stream. In practice, cooling air has to be exhausted through the gap at a significantly higher pressure than the mean static pressure in the field which is immediately downstream of the stator vane exits. This, in turn, leads to the use of a greater amount of cooling air than would otherwise be anticipated.
The reason for the use of higher pressure cooling air is the variation in static pressure which occurs in the field downstream of the stator vanes. Thus the static pressure varies so that in some parts of the field, it is significantly higher than in other parts. More specifically, the static pressure varies circumferentially in a roughly sinusoidal manner. Consequently the pressure of the cooling air exhausted through the gap must be higher than the highest pressure peak in the stator vane exhaust field.
It will be clearly apparent that such excessive use of cooling air is highly undesirable in view of the loss of mass flow which it provides to the overall engine cycle as well as the efficiency losses which it causes to the rotor blades which are immediately downstream of the annular clearance.